Optical-photoconductive reproducer utilizing insulative liquids



Sept 20, 1966 PA E. WRIGHT OPTICAL-PHOTOCONDUCTIVE REPRODUCER UTILIZING INSULATIVE LIQUIDS Filed May l, 1961 m f mw m mm 4 Ww E,

3,274,565 OPTICAL-PHOTOCONDUCTIVE REPRODUCER UTILIZING INSULATIVE LIQUIDS Paul E. Wright, Haddonield, NJ., assignor to Radio Corporation of America, a corporation of Delaware Filed May 1, 1961, Ser. No. 106,698 18 Claims. (Cl. 340-173) This invention relates to improved methods of electrostatic recording and more specifically to improved methods of and means for producing and pr-ojecting eidophor images.

An eidophor image is formed by the surface modula- -tion of a liquid lm by electrostatic forces. In some methods of reproducing images such as, for example, by means of Schlieren optics a surface modulated image is projected onto a viewing screen as a visible image. One method for preparing surface modulated images is described in U.S. Patent 2,644,938 to M. Hetzel et al. Briefly this method includes writing with an electron beam onto a liquid film to deform lthe surface thereof by electrostatic forces. The deformed or surface modulated liquid is utilized in a Schlieren optic system -to produce a projected visible image. A disadvantage of this method resides in the fact that the recording steps have to be carried out in vacuum.

It is a general object of this invention to provide improved means of and methods for producing eidophor images on films or slides and for utilizing such images.

It is another object of this invention to eliminate the need for recording within vacuum chambers in producing eidophor type images on films and slides.

A still further object is to provide improved methods of and apparatus for image reproduction wherein eidophor images are projected as light images on a viewing screen.

These and other objects and advantages are accomplished in accordance with this invention by producing a surface modulated liquid film on a photoconductive layer. Preferably the layer comprises a transparent organic photoconductive material. The layer has a resitivity in darkness of at least 109 ohm-centirneters. In light it has a resitivity at least two orders of magnitude less than the dark resistivity.

A thin film of insulating liquid such as, for example, a silicone o il, (dimethyl polysiloxane) having a viscosity of from 1 to 3 centipoises is applied to the photoconductive layer. This may be accomplished, for example, by flowing a silicone oil yover the photoconductive layer and a1- lowing it to drain. Roller coating, spray coating, or other equivalent methods may be employed.

An electrostatic charge image is then electrophotographically produced on the photoconductive layer bearing the thin film of silicone oil. This may be accomplished, for example, by producing an overall electrostatic charge on the surface of the layer and exposing that surface to incident radiation. Because of the difference in resistivity in darkness and in light the incident radiation substantially reduces -or removes the charge in the irradiated areas thereby forming the electrostatic image.

When -an electrostatic image is formed on the layer, the silicone oil gathers in .the charge areas of the image in a manner such that the silicone oil has a greater thickness in such areas than in those areas of the image which bear no charge or little charge. In lieu of forming such an image with silicone oil, any insulating liquid may be employed provided only that one be selected which will not deleteriously affect the photoconductive layer. In this regard some such liquids may be solvents for some photoconductive layer and the use of such solvents should be avoided.

vUnited States Patent O 3,274,565 Patented Sept. 20, 1966 In the foregoing method, it is not necessary that the silicone oil be applied to the photoconductive layer before that layer is electrostatically charged. The liquid may be applied subsequent to charging or it may be applied after the electrostatic charge image is produced on the surface of the layer. In any case, the desired eidophor image will be created.

In continuous reproducing system-s, the llayer may immediately pass to a projection station, which may comprise a Schlieren optical arrangement, where the surface modulated liquid image is converted into a visible light image.

This invention also includes apparatus for producing and utilizing the aforesaid eidophor images. In general the apparatus includes means for applying an insulating liquid to a photoconductive layer in a thin film, means for producing a substantially uniform electrostatic charge on the surface of the photoconductive llayer and means for producing a light image incident upon the layer. An optical projection station is also provided for converting the eidophor image into a projected light image.

Other objects and advantages appear from .the following `detailed description and the accompanying drawings wherein:

FIG. 1 is a schematic diagram in elevation of apparatus for recording and projecting surface modulated liquid images on a photoconductive layer in accondance with this invention.

FIG. 2, is an enlarged fragmentary schematic View in cross section illustrating the formation of the surface modulated liquid image on the layer of FIG. 1.

In practicing this invention, a thermoplastic photoconductive insulating layer, typified by the following example, is employed which preferably has a high degree of light transmissivity.

Example I 7 parts by weight of the leuco base of malachite green,-

bis-(4,4dimethylaminophenyl) phenyl methane.

4 parts by weight of a chlorinated paraffin such as, for example, Chlorowax 70, manufactured by the Diamond Alkali Co., Cleveland, Ohio.

20 parts by weight methyl ethyl ketone. j

The 7 parts of the leuco base of malachite green are dissolved in the polystyrene solution. The remaining materials are made into a second solution comprising chlorinated parain dissolved in the methyl ethyl ketone. The two solutions are then mixed together and coated on a suitable substrate such as, for example, conductive glass or metallized transparent film.

A preferred substra-te comprises high quality glass such as lantern .slide glass having on one surface thereof a vacuum deposited conductive film, for example, a film of tin chloride. The solution of Example I is applied to the conductive lm by well known techniques, for example, flow coating, dip coating or spin coating. The solvent is then evaporated from the coating on the slide to produce thereon a thin uniform photoconductive layer. When preparing a transparent slide for use in .the methods of this invention, it s preferred tha-t a small area of the conductive film be bared of photoconductive coating to provide means for electrically contacting or grounding the conductive film.

Another preferred substrate comprises high-melting film such as, for example, one sold under the trademark Mylar or Cronan A conductive surface can be readily produced cn such a film by vacuum deposition of a metal, for example, copper or aluminum, A photoconductive 'layer can be readily produced on the metallized lm by well known methods, for example, roll coating, flow coating or dip coating. Once the coating on the film is dried, a highly exible photoconductive film is provided.

Heat may be applied to a photoconductive layer on a rnetallized glass or film substrate to accelerate the drying thereof. Once dried, continued heating for from two to three minutes at a temperature of about 180 C. will produce a faint green `tint in the clear coating. When prepared in this manner, the layer on the substrate will have maximum photoconductive response to visible light of about 6300 A. and will have another response peak at about 4200 A. It will also have a relatively sharp thermoplastic transition point at Iabout 50 centigrade.

An eidophor image can be produ-ced on the layer of Example I by employing electrostatic printing techniques. One specific method includes the following steps:

(1) Liquid application-As described heretofore, a thin film of insulating liquid is applied to a photoconductive layer. Preferably the layer is one such as that set forth in Example I or as specified hereinafter. When the photoconductive layer is coated on lantern slide glass a liquid such as dimethyl polysiloxane may be poured over the surface thereof and allow,ed to drain. When the photoconductive layer is coated on flexible film, application of liquid thereto may be accomplished by roller coating or other equivalent methods.

(2) Electrostatic charging-With the substrate of Example I grounded or on a grounded plate, `and the thin film of liquid over it, a substantially uniform electrostatic charge is applied to the layer by passing thereover a corona generating device which includes at least one fine wire to which is applied a potential of about $5,600 volts. Some photoconductive layers are more eiciently charged with negative polarity, others with positive polarity. However, with respect to the photoconductive layers specifically described herein either polarity of charge may be employed with about equal efficiency. This step is carried out in darkness or in safe light to which the layer is insensitive.

(3) Exposing-With a photographic transparency resting on the layer, it is exposed for about 1 to 3 seconds to a 100 watt incandescent lamp spaced about 18 inches away to produce an electrostatic image consisting of charged areas on the layer which correspond to the dark areas of the transparency. Exposure time can be substantially decreased by increasing the light intensity. Projection exposure techniques can -be employed with equal facility.

As mentioned heretofore steps l, 2 and 3 outlined above may be carried out in any order to provide the desired eidophor image.

With the foregoing method, and employing a conductive glass slide coated with a thermoplastic photoconductive layer, an eidophor transparency can be prepared in a few seconds for use in a schlieren optic slide projector. For use in a continuous reproduction apparatus it is preferred to employ a flexible transparent metallized film instead of the glass slide.

The methods of this invention may be embodied in an apparatus such as that illustrated in FIG. 1. As shown in the figure an endless belt 11 is carriedon four rollers 13, 15, 17 and 19. One or more of these rollers, such as roller 13, may be driven by any suitable means, such as motor 14, to transport the belt 11 in the direction of the arrow 21. The endless belt 11 preferably comprises a suitable substrate, such as a copper coated, or aluminized transparent film 23 on which a photoconductive layer 25, such as that described in Example I has been laid down.

A thin lm of liquid is applied to the photoconductive layer 25 by means of the applicator 27. This applicator 27 may comprise, for example, a container 29 holding a supply of silicone oil 31 which is carried on to the photoconductive layer 25 by means of the reverse roller 33.

A substantially uniform electrostatic charge is produced on the photoconductive layer 25 as it passes lunder a corona generating source 35. This source 35 may cornprise an array of line parallel Wires 37 supported in a shield 39. The wires 37 are connected to a high voltage source 41 and 15000 or more volts are applied to the wires 37 to generate thereby corona for charging the photoconductive layer 25. Charging will be enhanced if, during generation of corona, the metallized film 23 is at ground along potential. This can be easily accomplished by leaving a strip along one edge `of the transparent iilm 23 bare of the photoconductive layer 25 and maintaining a grounded conductive wheel 40 in conductive Contact with the bared strip of the transparent film 23.

The photoconductive layer 25 next passes to an exposure station where it is exposed to a light image from a cathode ray tube 42 or a flying spot scanner connected to a suitable receiver 43. The desired exposure can also be obtained from an ordinary film or slide projector. The electrostatic charge in all areas of the photoconductive layer struck by light is substantially reduced or dissipated. In this way, a latent electrostatic image is formed on the photoconductive layer 25 and an eidophor image created from the silicone oil on the layer 25. Creation of the eidophor image is illustrated in much exaggerated form in FIG. 2. In areas where charges remain after exposure to light the liquid film 31 is bulged to fonm the eidophor image as illustrated at 32. FIG. 2 also illustrates how a strip 23 of the metallized surface of the transparent lm 23 can be left bare to provide a contact surface for the grounded Wheel 40. l

Upon completion of exposure from the cathode ray tube 42 an ei-dophor image is produced on a photoconductive layer 25. This image is then transported on the layer to a schlieren optic projector 45. This projector will include, for example, a point light source 47, a slotted baffle 49, a condensing lens 51, a grid of bars 53 in registry with the slots in the -baflie 49 and a projection lens 55. There are several Well known schlieren optic systems which could be employed in the apparatus of FIG. l. For a more complete description of schlieren optic projection, reference is made to U.S. Patent 2,644,938 to M. Hetzel et al. In the projector 45, the eidophor image on the photoconductive layer 25 causes a visible light image to be projected onto a viewing screen 57.

If it is only desired to view the image on a screen 57 momentarily the nature of the light source in the schlieren projector 45 is in no way critical. However, if the image is to be viewed for more than about 4 seconds light passing through the photoconductive layer 25 will, if the layer is photoconductively sensitive to that light, discharge the layer and destroy the eidophor image. In such cases, the light source should be selected to be one emitting light to which the layer is not sensitive. In the case of the photoconductive layer of Example I, a light source emitting little or no red light will provide for viewing times of several minutes or more.

As the endless belt is moved from the schlieren projector and passes the liquid applicator 27 and the corona source 37, the electrostatic image on the photoconductive layer 25 will be obliterated and in consequence the eidophor image as well. In some instances, past light history of the layer 25 may result in incomplete obliteration of the electrostatic image as it passes under the corona source 35. This may result in an eidophor ghost image being retained on the layer 25 when a new eidophor image is produced thereon. Any such difficulty can be obviated by heating the layer, subsequent to schlieren projection, to a temperature of from about 40 to 50 degrees centigrade. Such heating will effectively erase any elec-trostatic image from the photoconductive layer 25. In the apparatus of FIG. l, heating is provided for by employing resistance heating elements 59 connected'to a source (not shown) of heating current. Erasure of electrostatic images from the photoconductive layer 25 occurs as the layer 25 passes between the heating elements 59. In the alternative, erasure could be accomplished by replacing the heating elements 59 with a strong light source emitting light to .which the layer 25 is sensitive.

Once the surface modulations have been erased, the endless belt is ready to be recycled to produce another Schlieren projected image.

In operating the device of FIG. 1, movement of the endless belt 11 is desirably controlled by a start-stop cycling means 61. Such control is desirable in the case where images projected onto the screen 57 are viewed for different periods of time than required for exposure of the electrostatic images. In addition, such control can provide `for regulation of exposure from the cathode ray tube 42. The drive motor 14 thus provides frame by frame movement of the endless belt 11. During such movement, the endless belt 11 passes under the charging wires 37 and the photoconductive layer 25 becomes electrostatically charged. Since this layer 25 is a good insulator in darkness, it will retain its charge until struck by light from the projector 43.

Under control yof the cycling means 61 the endless belt 11 is stopped during exposure to a light image from the projector 43. During this exposure, an image being viewed on the screen 57 may be retained for a time in excess of that required for complete exposure of the frame which is at rest under the cathode ray tube 42. To prevent over-exposure, the cycling means 61 is coupled to a switch 62 in the control circuit of the cathode ray tube 42. Again, due to the high dark resistivity of the photoconductive layer 25, the electrostatic image produced thereon by exposure will be retained for a considerable length of time.

In lieu [of the combination of resinous materials, polystyrene and chlorinated parain, set forth in Example I, any one of many other resinous `materials or combinations thereof may be employed in the thermoplastic photoconductive layers described herein. Suitable resinous materials include the following:

(1) Chlorinated parains, such as Chlorowax 70,

Diamond Alkali Co., Cleveland, Ohio'.

(2) Polyvinyl chloride.

(3) Polyvinyl chloride copolymers, such as Vinylite- VAGH, 91% vinyl chloride, 3% vinyl acetate and 6% vinyl alcohol; VYCM 91% vinyl chloride and 9% vinyl acetate; VMCH 86% vinyl chloride, 13% vinyl acetate, and

1% dibasic acid.

(4) Polystyrene.

(5) Styrene butadiene copolymers such as Pliolite S-S, The Goodyear Tire and Rubber Co., Akron, Ohio; Piccotex 120, Pennsylvania Industrial Chemical Co., Clainton, Pa.

(6) Hydrocarbon resins such as Piccotex 120, Pennsylvania Industrial Chemical Co., Clairton, Pa. (7) Acrylates and acrylic copolymers, such as Acryloid A-lOl, Rohm and Haas Co., Philadelphia, Pa.

(8) Epoxy resins, such as Epon 1002, Shell Chemical Co.,

Houston, Tex.

(9) Thermoplastic hydrocarbon terpene resins, such as Piccolyte S-135, Pennsylvania Industrial Chemical Co.

Various combinations of resinous materials can be employed to provide enhanced exibil-ity in the thermoplastic layers, for example, mixtures of polyvinyl chloride with chlorinated parafns or hydrocarbon terpene resins will provide highly flexible layers.

'Ilo provide a substantially transparent photoconductive layer a dye-intermediate is selected which is soluble in the selected resin. The leuco base of malachite green set forth in Example I is only one of a large class of suitable dye intenmediates.

It has the formula:

In general, the suitable dye .intermediates have the basic formula:

wherein R1 and R2 are selected from the class consisting of mono-alkylamino, di-alkylamino, mono-arylamino, and alkyla-rylamino; X is selected from the class consisting of H,

wherein R3 is selected from the class consisting of H, OH, CH3, OCI-H3, R1 and Gi-@et CH, i

(3) Bis-(4,4eimethylaminophenyl)-4" methoxyphenyl methane:

ogs l /CH3 /N-@tt-@N C H3 H \CH3 (4) Bis-(4,4-dimethylamin'ophenyl)-4" hydroxyphenyl methanez /CH3 CH H \CH3 (5) Bis-(4,4'dimethylaminophenyl) methane:

CH3 H CH3 /N-O-i-@N/ CH3 IL \CH3 1 s (6) 4,4bis(dimethylamino) benzophenone (Michlers (13) Bis-(4,4methylaminophenyl) 4"-tolyl methane:

ketone): CH3 01er3 o CH3 WCW-@Ni 5 Ca CH3 (7) Bis-(4,4dimethylarninophenyl) 4"-to1yl methane: H H

l0 (14) Bis (4,4 dimethylaminophenyl) 2,4" dimethoxyphenyl methane:

ogs /CH3 0 CH3 Cg II \CH3 15 (8) Bis-(4,4'-ethyl-benzylaminophenyl) phenyl methane: 00H3 CH3 CH3 02H5 (32H5 20 Ca III \CH3 l Bis-(4,4'dimethylaminophenyl)-2,4Xylyl meth- /N- ?-@N\ |JH2 H (f1-12 CH3 (9) Bis-(4,4'dimethy1aminopheny1) 2" 4" dihydroxycg@ /CH3 phenyl methane: /N f-C `N\ 0H CH3 H CH3 16) Bis-(4,4phenylaminophenyl) 4ethy1amin0pheny1 methane: OH (IDEES CH3 CH3 N Ca \CH3 (10) Bis (4,4 morpholiuophenyl) phenyl methane: 40 H H @iQ-001e@ Ii l-Iz Egg, (17) Bis (4,4methylaminophenyl)4hydroxypheny1 O/ Q1-@4 l 0H H g I 1 1 Tris-(4,4,4"-phenylaminophenyl) methane: 50

H H cH-llr-O-C-Q-IlI-Cm I (l)CH3 (19) Bis-(4,4methylaminopheny1)-4"toly1 methane:

CzHs 03H5! )LCH-@M CHQsQlCHa 9 (20) Bis (4,4 methylaminophenyl)-2,4"-dihydroxy phenyl methane:

(2l) Bis (4,4 methylaminophenyl)-2,4dimethoxyphenyl methane:

OCHa

(22) Bis-(4,4-methylaminopheny1)2,4-xylyl methane:

It t

(23) 4,4-bis(ethyl-benzylamino) benzophenone:

(.24) 4,4bis-(ethyl-phenylamino) benzophenone:

(25) Bis (4,4' ethyl-benzylaminophenyl)-2,4"dihydroxyphenyl methane:

, 10 (26) Tris- (4,4,4"-ethylphenylaminophenyl) methane:

CZHr-N CzHs /CzHs Photoconductive compositions are conveniently prepared, for example, by dissolving a quantity of .the resinous material in a suitable solvent such as, for example, methyl ethyl ketone, toluene or mixtures thereof and, when the resinous material is completely dissolved, adding to the solution a quantity of the dye intermediate. The proportion of dye intermediate to resinous material may vary over a wide range. The choice of resinous material as well as the dye intermediate can change the optimum ratio for a given use. In many instances, it is desirable that a photoconductive layer or coating be as transparent as possible. For such purposes 0.8 part by weight or less of dye intermediate for each part by weight of resinous material can be employed. For some purposes, the color of a photoconductive `film or coating may not be of major concern. For such purposes, up to 1.4 parts by weight or more of dye intermediate for each part by Weight of resinous material may be employed. The solubility of a particular dye intermediate in a particular resin should also be taken into consideration. In some instances, if a solution is prepared containing too much dye intermediate the excess thereof Will, upon drying, crystallize out of solution which generally is undesirable.

Further illustrations of compositions which can be used to form transparent photoconductive layers exhibiting thermoplastic properties which are useful in the same manner as described in connection with Example I include the following solutions:

Example Il 2.5 parts by weight `bis-(4,4-dimethylaminophenyl) phenyl methane, and

5.0 parts by weight of a styrene-butadiene copolymer such as, for example, Pliolite S-5B by the Goodyear Tire and Rubber Co., Akron, Ohio, dissolved in 42.0 parts by weight of methyl ethyl ketone.

A layer madel from this solution has a softening temperature of about 55 to 57 C.

A layer made from this solution has a softening temperature of about 56 to 58 C.

Example I V 1.5 parts by weight of bis-(4,4'-dimethyl-aminophenyl) phenyl methane, and 5 .0 parts by weight of styrene-butadiene copolymer (Pliolite S-S dissolved in 42.0 parts by weight of methyl ethyl ketone.

A layer made from this solution has a softening point of about 54 to 56 C.

1 1 Example V 1.0 part by weight of tris-(4,4,4-dimethylaminophenyl) methane, and

5.0 parts by weight of styrene-butadiene copolymer (Pliolite S-5 dissolved in 42.0 parts by weight of methyl ethyl ketone.

A layer made from this 4solutionhas a softening point of about 85 to 87 C.

Example VI 1.0 part by weight of tris-(4,4,4"-dimethylaminophenyl) methane, and l,

5.0 parts by weight of a hydrocarbon resin such as, for example, Piccotex P120, Pennsylvania Industrial Chemical Corp., Clairton, Pa., and

1.6 parts by weight of a polyvinyl chloride copolymer such as, for example, GEON 400X-l10, B. F. Goodrich Che'mical Co., Akron, Ohio, dissolved in 50.6 parts by'weight of methyl ethyl ketone.

A layer made from this solution has a softening point of about 50 to 52 C.

Example VII 2.5 parts by weight of bis-(4,4dimethylaminophenyl) phenyl methane, and

5.0 parts by weight 0f a high styrene copolymer such as, for example, Marbon 9200 LLV, Marbon Chemical Co., a division of Borg-Warner Corp., Gary, Ind., and

1.2 parts by Weight of a vinyl chloride copolymer such as, for example, Vinylite VYCM, Union Carbide Plastics Co., a division of Union C-arbide Corp., New York, N.Y., dissolved in 72.0 parts by weight of methyl ethyl ketone.

A layer made from this solution has a softening point of about 50 C.

Example VIII 1.0 part by weight of tris-(4,4,4"-dimethylaminophenyl) methane, and

5.0 parts by Weight of a high styrene copolymer (Marbon M-1100 TMV), and

1.6 parts by weight of a polyvinyl chloride copolymer GEON 400X-l), dissolved in 50.0 parts by weight of methyl ethyl ketone.

A layer made from this solution has a softening point of about 48 to 50 C.

Example IX 1.0 part by weight of bis-(4,4-dimethyl-aminophenyl) phenyl methane, and

5.0 parts by weight of a hydrocarbon resin (Piccotex P-100), and

1.6 parts by weight of a vinyl chloride copolymer (Vinylite VMCH) dissolved in 50.0 parts by weight of methyl ethyl ketone.

A layer made from this solution has a softening point of about 40 C.

Example X 1.0 part Iby weight of tris-(4,4',4"-dimethyl-aminophenyl) methane, and

5.0 parts by weight of a polystyrene resin such as, for example, Styron PS-2, The Dow Chemical Co., Midland, Mich., and

1.6 parts by Weight of a vinyl chloride copolymer (Vinylite VMCH), dissolved in 50.0 parts by weight of methyl ethyl ketone.

A layer made from this solution has a softening point of about 52 C.

Example XI 1.0 part by weight of tris-(4,44"-dimethylaminophenyl) methane, and

5.0 parts by weight .ofL polystyrene resin (Styron PS-2),

and

12 1.6 parts by weight of polyvinyl chloride copolymer (GEON 400X-l10), dissolved in 50.0 parts by weight of methyl ethyl ketone.

A layer made from this solution has a softening point of about 47 C.

Example XII 1.0 part by weight of tris- (4,4,4-dimethyl aminophenyl) methane, and

5.0 parts by weight of a hydrocarbon resin (Piccotex P-), and

1.6 parts by weigh-t of a vinyl chloride copolymer (Vinylite VMCH), dissolved in 50.0 parts by weight of methyl ethyl ketone.

A layer made from this solution has a softening point of about 63 to 65 C.

Various modifying agents may be added to the forelgoing compositions to vary the physical properties or appearance thereof provided they do not interfere with the electrical properties. |Flexibility can `be enhanced, for example, by including in composition, such `as that of Example I, a small amount of a plast-icizer, such as, for example, tricresyl phosphate, butyl phthalylbutyl-glycolate, tris-(2,3-dibromo-propyl) phosphate, or di-(2ethyl hexyl) phthalate. Such a composition can be coated on a flexible substrate or can be formed into self-supporting flexible films. A self-supporting film may -be produced by flow-coating a mirror-huish metal plate with the cornposition to form a photoconductive coating on the plate. The coating i-s then physically stripped from the plate and thus provides a self-suporting photoconductive film. Additional solvents can also 'be added, `such as, for example, toluene to produce the desired coating thickness of the dry iinished thermoplastic photooonductive layer.

When a compositi-on is prepared wherein a dye intermediate is `dissolved in a non-halogenated resin, enhanced photoconductive response can often be obtained or at least ensured Iby including in the composition at least a trace amount of a compatible non-volatile halogenated compound such as, -for example, tris- (2,3-dibromopropyl) phosphate or lany compatible chlorinated hydrocarbon.

Many of the compositions contemplated herein, when coated on a substrate or formed in-to a film, may have a tendency to form color which may be vundesirable under some circumstances. Color formation in a film or coating can *be substantially retarded =by including in the compositions a small amount of stabilizer for the dye intermediate thereof. A speciic example of a suitable stabilizer is one having the formula (Thermolite 20, Metal and Thermit Corp., Rahway, NJ.) Other materials such -as pyrocathechol, 2-hydroxy-4-methoxy benzophenone, `and 2,2'dihydroxy4rnethoxy benzophenone may also be used. Some compositions includin'g such a stabilizer will remain Isubstantially colorless for a considerable time unless subjected to intense ultraviolet radiation.

What is claimed is:

1. A method of image reproduction employing a photoconductive insulating layer; Isaid method comprising applying a thin film of insulating liquid to said layer, electrophotographically producing a latent electrostatic image on said layer, an-d illuminating Isaid layer to convert the surface modulated liquid image produced hy said electrostatic image into a projected visible image.

2. The method of claim 1 wherein said photocondnctive insulating layer is substantially tnansparent and the said converting of said surface modulation image includes the projecting of light through said layer.

3. A method of image reproduction employing a photoconductive insulating layer having lminimum photoconductive sensitivity to at least one hand of wavelengths; said method comprising applying a thin hlm of insulating |13 liquid to said layer, electrophotographical-ly producing a latent electrostatic image on said layer, and illuminating said layer with light to which said layer is relative-ly insensitive to convert the 4surface modulated liquid image produced by 'said electrostatic image into 'a 'visible image.

4. The method of claim 3 where-in said photoconductive insulating layer is relatively transparent and said surface modulated image is converted into a visible image by projecting said light through said layer.

5. In la method of image reproduction employing a photoconductive insulating layer comprising an organic resinous material having dissolved therein a dye intermediate, said layer having a resistivity in darkness of at least 109 ohmcentimeters :and a resistivity when irradiated of at least two orders of magnitude less than said resistivity in darkness; the steps -of applying a thin lm of insulating liq-uid to said Ilayer and electrophotographically producing fa latent electrostatic .image on said layer to pro-duce on said liquid a su-rface modulation image.

6. A method of image reproduction employing a substantially transparent photoconductive layer comprising an organic resinous material ha'ving dissolved therein a dye intermediate, said layer having -a resistivity in darkness of at least 109 ohm-centimeters and a resistivity when irradiated of at least two orders of magnitude less than said resistivity in darkness; said method comprising apply-ing a th-in iilm of insulating liquid to said layer, applying 4a substantially uniform electrostatic 'charge to said layer, exposing said layer to a -light :image to produce thereon a latent electrostatic image, and converting the surface modulated liquid image formed Iby said electrostatic image into a visible image by schliefen projection.

7. The method of claim 1 wherein said photoconductive layer is relatively insensitive to light of at least one band of wavelengths and said light is employed during said Schlieren lprojection.

8. Image reproduction appara-tus employing a photoconductive insulating layer; said apparatus comprising means for applying an insulating liquid to s-aid layer as a thin iilm, means 'for producing a substantially uniform electrostatic charge on said layer, means for exposing said layer to a radiation image to pro-duce thereon a latent electrostatic image, and means for illuminating said layer to convert the surface modulated rliquid image formed by said electrostatic image into a visible light image.

9. rIlhe apparatus of claim 8 including means for sub stantially uniformly flooding said layer with radiation to obliterate said latent electrostatic image.

10. The Iapparatus of claim 8 'wherein said means for illuminating said layer comprises a Schlieren optic projector.

11. Image reproduction apparatus comprising means for ymoving a photoconductive insulating layer along a predetermined path, means `adjacent said path for applying \an insulating liquid to said layer as a thin iilm, means for producing a substantially uni-form electrostatic charge on said layer, means for exposing sai-d layer t-o a light image to produce thereon a -lfatent electrostatic image,

and Schlieren optic means for converting the surface Imodulated liquid image formed fby said electrostatic image into a projected light image.

12. The apparatus of claim 11 including means for substantially uniformly 4flooding said layer with light to obliterate said latent electrostatic image.

13. Image reproduction appanatus comprising a substantially transparent elongated layer of photoconductive insulating material, means Ffor transporting said layer along a predetermined path, means adjacent said path for applying insulating liquid to -said Ilayer as a thin iilm, corona generating means for produc-ing a substantially uniform electrostatic charge on said layer, means for projecting a light image onto said layer to produce thereon a latent electrostatic image, and Schlieren optic means for illuminating said layer to convert the surface modulated im-age formed by said latent electrostatic image into a projected light image.

14. The apparatus of claim 13 wherein said layer comprises Ian endless loop.

15'. The apparatus of cl-aim 13 wherein said photoconductive insulating Imaterial is relatively insensitive to light of at least one lban-d `of ywavelengths and said light is employed to illuminate said layer in said lSchlieren optic means.

16. The apparatus o'f claim 13 including means for substantially uniformly irradiating said layer to obliterate said latent electrostatic image.

17. The apparatus of claim 13 wlherein said photoconductive insulating material comprises an organ-ic resinous material having dissolved therein a dye intermediate and having a resistivity in darkness of at. least 109 ohm-centimeters and a resistivity when irradiated of at least two orders of magnitude less than said resistivity in darkness.

18. -In image reproduction apparatus the combination comprising a photoconductive insulating layer comprising an organic resinous material having dissolved therein a dye intermediate, said layer having a resistivity in darkness yof at least 109 Iohm-centimeters and 'a resistivity when irradiated of at least -tw-o orders of magnitude less than ysaid resistivity in darkness; means for applying an insulating liquid to said layer in a thin iilm, means for producing a substantially uniform electrostatic charge on said layer, and means lfor exposing said layer to a radiation image `to produce thereon a latent electrostatic image and to 'form on said thin iilm a surface modulation image.

References Cited by the Examiner UNITED STATES PATENTS 2,391,451 12/1945 Fischer 178-6.6 2,901,374 8/19591 Gundlach 346-74 2,996,573 I8/1961 yBarnes 17 8--6.6 3,055,006 9/ 1962 Dreyfoos 346-74 3,113,179 12/1963' Glenn 178-6.6

BERNARD KONICK, Primary Examiner. E. SAX, IRVING L. SRAGOW, Examiners.

A. I. DUNN, R. J. MCCLOSKEY, T. W. FEARS,

Assistant Examiners. 

13. IMAGE REPRODUCTION APPARATUS COMPRISING A SUBSTANTIALLY TRANSPARENT ELONGATED LAYER OF PHOTOCONDUCTIVE INSULATING MATERIAL, MEANS FOR TRANSPORTING SAID LAYER ALONG A PREDETERMINED PATH, MEANS ADJACENT SAID PATH FOR APPLYING INSULATING LIQUID TO SAID LAYER AS A THIN FILM, CORONA GENERATING MEANS FOR PRODUCING A SUBSTANTIALLY UNIFORM ELECTROSTATIC CHARGE ON SAID LAYER, MEANS FOR PROJECTING A LIGHT IMAGE ONTO SAID LAYER TO PRODUCE THEREON A LATENT ELECTROSTATIC IMAGE, AND SCHLIEREN OPTIC MEANS FOR ILLUMINATING SAID LAYER TO CONVERT THE SURFACE MODULATED IMAGE FORMED BY SAID LATENT ELECTROSTATIC IMAGE INTO A PROJECTED LIGHT IMAGE. 